The present invention generally relates to communication systems and more particularly to a hybrid circuit utilizing a 20 Hz threshold detector in which the hybrid circuit provides conduction or isolation between selected paths of two-wire and four-wire lines.
Hybrid systems are well known in the communication field. They are used to connect a two-wire bidirectional balanced transmission path with a four-wire path. If the hybrid system does not have proper canceling parameters, undesirable signals appear on specified lines. For instance, at the four-wire input/output terminals, a signal introduced to the hybrid system at the input terminals must not appear at the output terminals. Furthermore, a signal introduced to the hybrid system at the two-wire input/output terminal must appear at the output terminals but not at the input terminals of the four-wire input/output. The purpose of the system is to have the signals introduced to the system on the input terminals of the four-wire input/output end transmitted only to the two-wire input/output end and the signals introduced at the two-wire input/output end transmitted only to the two output lines of the four-wire input/output end. In a system providing 20 Hz signalling at the two-wire end, isolation is provided from both the input and output of the four-wire end.
Special balancing transformers and more recently solid state devices have been used to provide the appropriate canceling. None of the prior art, however, have accomplished a system as effectively simplistic and capable of miniaturization as the present invention.